Planar transmission line low-pass filters with absorptive matrix and method for forming the same

ABSTRACT

Described is a method for forming a planar transmission line low-pass filter and a resulting filter. The method comprises several acts, including using lithographic processes and a castable polymer with absorptive matrix as a spin-on dielectric to form the planar transmission line low-pass filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and is a non-provisional patentapplication of U.S. 63/027,786, filed on May 20, 2020, the entirety ofwhich is hereby incorporated by reference.

BACKGROUND OF INVENTION (1) Field of Invention

The present invention relates to low-pass filters and, morespecifically, to a planar transmission line low-pass filter withabsorptive matrix.

(2) Description of Related Art

The present disclosure is generally directed to a planar transmissionline filter. The idea of using planar transmission lines, in variousgeometries, as electromagnetic (EM) filters (low-pass, high-bass,band-pass, band-stop) is a long-established practice.

By way of example, some early work suggested the idea of using thin-filmprocesses to construct dissipative RF filters for cryogenicenvironments. In the work of Vion et al, it is the skin effect of along, lossy conductor that provides the loss, rather than the dielectricin a transmission line (see Vion, D., Orfila, P. F., Joyez, P., Esteve,D., & Devoret, M. H. (1995). “Miniature electrical filters for singleelectron devices,” Journal of Applied Physics, 77(6), 2519-2524, theentirety of which is incorporated herein by reference). An improvementon the above design was described by le Sueur et al. (2006).“Microfabricated electromagnetic filters for millikelvin experiments”,Review of scientific instruments, 77(11), 115102, the entirety of whichis incorporated herein by reference. However, some deficits remained.The work of Santavicca et al. showed that castable Eccosorb couldoperate as a lossy dielectric in a cryogenic transmission line filter(see Santavicca, D. F., & Prober, D. E. (2008). “Impedance-matchedlow-pass stripline filters.” Measurement Science and Technology, 19(8),087001, the entirety of which is incorporated herein by reference). Intheir work, the resulting filter cavity was machined from a solid blockof copper and the transmission line was hand soldered in. The cavity wasthen injected with the dielectric by-hand with a syringe, which isincredibly time intensive and prone to error.

A characterization and tuning of the Santavicca (2008) design was laterdescribed in Slichter, D. H., Naaman, O., & Siddiqi, I. in “Millikelvinthermal and electrical performance of lossy transmission line filters,”Applied Physics Letters, 94(19), 192508 (2009), the entirety of which isincorporated herein by reference.

Yet another implementation of the Santavicca (2008) design was describedby Wollack, E. J., Chuss, D. T., Rostem, K., & U-Yen, K., in “Impedancematched absorptive thermal blocking filters,” Review of ScientificInstruments, 85(3), 034702 (2014), the entirety of which is incorporatedherein by reference.

A totally different type of filter was also devised that attempts tosolve the same problem using miniaturization techniques, showing thatbetter dissipative filters for cryogenic measurements is something thecommunity continues to chase. Said filter was described by Longobardi,L., Bennett, D. A., Patel, V., Chen, W., & Lukens, J. E., in “Microstripfilters for measurement and control of superconducting qubits,” Reviewof Scientific Instruments, 84(1), 014706 (2013), the entirety of whichis incorporated herein by reference.

Further, a recent review article on the state of cryogenic RF filteringwas published by Thalmann, M., Pernau, H. F., Strunk, C., Scheer, E., &Pietsch, T., in “Comparison of cryogenic low-pass filters. Review ofScientific Instruments,” 88(11), 114703 (2017), the entirety of which isincorporated herein by reference. Yet another recent articlecharacterizing some consumer off-the-shelf (COTS) filters for cryogenicperformance below 1 gigahertz (GHz) was published by Zavyalov, V. V.,Chernyaev, S. A., Shein, K. V., Shukaleva, A. G., & Arutyunov, K. Y., in“Examination of cryogenic filters for multistage RF filtering inultralow temperature experiments,” In Journal of Physics: Conferenceseries (Vol. 969, No. 1, p. 012086), IOP Publishing (2018, March), theentirety of which is incorporated herein by reference.

Since Santavicca (2008) published their original paper on using Eccosorb(a commercially available, castable polymer impregnated with magneticnanoparticles) in cryogenic microwave filters, there has been a generaldrive to establish a process to make the filters smaller and to automateproduction via modularization as much as possible. New publications andpatents regarding filters of this type all address one of these twoimprovements, but all designs so far utilize the method described bySantavicca (2008). The current designs machine a macroscopic cavity andthen inject castable Eccosorb. In doing so, a single filter takes up avolume of order a few cubic centimeters. Notably, none of the prior artis capable of miniaturization by applying lithographic microfabricationtechniques to integrate filters directly at the chip level where theyare needed

Thus, a continuing need exists for a planar transmission line low passfilter formed through lithographic microfabrication techniques tointegrate filters directly at the chip level where they are needed,allowing for improvements over the prior art, such as generation of asingle wafer that contains many (e.g., dozens) individual one centimetersquare filter chips, each of which could have hundreds of discretetransmission line filters.

SUMMARY OF INVENTION

This disclosure provides a method for forming a planar transmission linelow-pass filter and a resulting filter. The method comprises severalacts, including using lithographic processes and a castable polymer withabsorptive matrix as a spin-on dielectric to form the planartransmission line low-pass filter. Using lithographic processes and acastable polymer with absorptive matrix as a spin-on dielectric to formthe planar transmission line low-pass filter comprises acts of: placinga layer of conducting material on a wafer substrate; applying and curinga polymer onto the layer of conducting material; adding a photoresist tothe polymer; applying a hard mask such that the hard mask covers thephotoresist and covered portions of the polymer, leaving exposedportions of the polymer exposed; removing the exposed portions of thepolymer; removing the hard mask to expose the photoresist and coveredportions of the polymer; and removing a portion of the conducting layer.

In another aspect, the present disclosure provides dissipative striplinetransmission line filter, comprising: a microstrip ground plane, themicrostrip ground plane being a patterned conducting layer formed on awafer substrate; a first polymer dielectric layer having magneticnanoparticles formed on the ground plane; a microstrip signal lineformed on the polymer dielectric; a second polymer dielectric havingmagnetic nanoparticles formed on the signal line and first polymerdielectric layer; and a second ground plane being a patterned conductinglayer formed on the second polymer dielectric layer.

In another aspect, at least one of, or both, of the first and secondpolymer dielectric layers have a thickness between 0.1 and 1000 microns.

In yet another aspect, each of the first and second polymer dielectriclayers have a thickness between 1 to 100 microns.

Further, the ground plane has a thickness between 0.1 and 10 microns.

In another aspect, the ground plane has a thickness between 0.5 to 3microns.

In yet another aspect, the microstrip signal line has a width between0.1 and 1000 microns. In another aspect, the microstrip signal line hasa width between 1 to 100 microns.

Finally, the present invention also includes a method for using theinvention as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will beapparent from the following detailed descriptions of the various aspectsof the invention in conjunction with reference to the followingdrawings, where:

FIG. 1A is an illustration depicting a process for forming atransmission line low-pass filter according to various embodiments ofthe present invention;

FIG. 1B is an illustration of a microstrip transmission line filterformed according to various embodiments of the present invention;

FIG. 1C is a (not-to-scale) cross-sectional view illustration of Step ofFIG. 1A, wherein some dimensions have been reduced or exaggerated forclarity;

FIG. 2 is a graph illustrating a simulation of the filter as depicted inFIG. 1B, showing a loss of 10 dB/cm at 1 GHz and a loss of 100 dB/cm at10 GHz;

FIG. 3 is a graph illustrating a simulation of the filter as depicted inFIG. 1B, showing a good reflection coefficient for frequencies up to 5GHz;

FIG. 4A is an illustration of a stripline transmission line filterformed according to various embodiments of the present invention;

FIG. 4B is a (not-to-scale) cross-sectional view of the illustration asdepicted in FIG. 4A, wherein some dimensions have been reduced orexaggerated for clarity;

FIG. 5 is a graph illustrating a simulation of the filter as depicted inFIG. 4A, showing a loss of 3.2 dB/cm at 10 GHz;

FIG. 6 is a graph illustrating a simulation of the filter as depicted inFIG. 4 showing a good reflection coefficient for frequencies up toroughly 15 GHz;

FIG. 7 is a spin-speed lookup plot used to estimate a spin speed andtemperature, depicting a target thickness and viscosity of the spunpolymer;

FIG. 8 is a scanning electron microscope (SEM) cross-sectional imagedepicting a Si wafer with a polymer spun onto the Si wafer;

FIG. 9A is a graph illustrating cross-sectional results from an SEM scanof a spun polymer (different from that of FIG. 8 ); and

FIG. 9B is a table summarizing two-dimensional surface parameters of thespun polymer of FIG. 9A.

DETAILED DESCRIPTION

The present invention relates to low-pass filters and, morespecifically, to a planar transmission line low-pass filter withabsorptive matrix. The following description is presented to enable oneof ordinary skill in the art to make and use the invention and toincorporate it in the context of particular applications. Variousmodifications, as well as a variety of uses in different applications,will be readily apparent to those skilled in the art, and the generalprinciples defined herein may be applied to a wide range of aspects.Thus, the present invention is not intended to be limited to the aspectspresented, but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112(f). In particular, the useof “step of” or “act of” in the claims herein is not intended to invokethe provisions of 35 U.S.C. 112(f).

Before describing the invention in detail, first an introductionprovides the reader with a general understanding of the presentinvention. Finally, specific details of various embodiment of thepresent invention are provided to give an understanding of the specificaspects.

(1) Introduction

The present disclosure is directed to a planar transmission line filterfabricated on a wafer substrate (e.g., sapphire, silicon, magnesiumoxide (MgO), etc.). The filter is fabricated using micro/nanofabrication procedures, with a dielectric that includes a spun-on andsubsequently patterned castable polymer (e.g., epoxy, resin, or glass)containing magnetic nano-particles. The filter can be implemented tooperate as a filter in a variety of applications, such as a coplanarwaveguide (CPW), a microstrip filter, a stripline filter, a coplanarstrip (CPS) filter.

A chief objective of this filter is to act as a dissipative (as opposedto a reflective) filter for out of-band radiation. One particularlyuseful feature of such a filter is that it operates perfectly well atcryogenic temperatures down to absolute zero. Furthermore, the conductorelement of the transmission line of such a cryogenic manifestation couldbe made from a superconductor, thus ensuring that only the controlleddielectric material accounts for loss (i.e., the loss is 100% tailored).The attenuation of the dielectric is such that it presents anexponential roll-off with increasing frequency that persists into theoptical regime of electromagnetic radiation. This is in contrast toreflective lumped-element filters that stop working at frequencies suchthat the associated wavelength is of order the same size as thecomponent (a few tens of GHz, typically).

A unique aspect provided by the present disclosure is the use ofmicro-fabrication procedures to integrate and pattern a spin-on polymerwith embedded magnetic particles as a dielectric in planar transmissionline filters that may be used as stand-alone components, or integrateddirectly with existing integrated-circuit fabrication methods. There areseveral advantages to this technique, including:

-   -   1. The use of micro-fabrication technologies for construction        dramatically reduces the size scale of the filter over current        approaches that effectively require by-hand assembly and        fabrication in a one-at-a-time fashion.    -   2. Utilizing a micro-fabrication wafer-based approach means more        filters in a given footprint (on of order 100 per centimeter),        enhanced uniformity, and the ability to use proven fabrication        tools for automation that result in economies of scale. In other        words, this approach enables massively improved scalability and        mass-production.    -   3. By using the same fundamental processes by which integrated        circuits are fabricated, such filters may be integrated directly        on the chip with the device under test, rather than as        stand-alone modules.    -   4. An additional benefit of this method of fabrication that        applies to the cryogenic use of such filters is the realization        of enhanced thermalization at cryogenic temperatures, resulting        in further improved performance.

As can be appreciated by those skilled in the art, the filter asdescribed herein is broadly applicable to any device or measurementrequiring low-pass filtering, and is especially useful for sensitivemeasurements conducted at cryogenic temperatures. Indeed, some suchmeasurements are not possible at all without some sort of cryogenicdissipative filter. Applications of such a filter include the cryogenichigh-frequency measurement of quantum bits which necessitate suchfiltering.

(2) Specific Details of Various Embodiments

As noted above, the invention is the application of microfabricationtechniques to the creation of electromagnetic low-pass filters made oflossy absorbing material. As an example, such a lossy, microwave- andinfrared-absorbing material might be Eccosorb® CR from Laird™ (locatedat 8 Pengfeng Rd, Songjiang Qu, Shanghai Shi, China), in which magneticnanoparticles are embedded in a polymer. That example polymer isprovided, uncured, as two liquids, which are later combined before useand curing.

As shown in FIG. 1A, the process involves placing a patterned conductinglayer 102 (e.g. copper) on a wafer substrate 100 (e.g. silicon). Afterwhich a patterned layer of absorbing material is applied. The patternedlayer of absorbing material is applied using an etching technique, asdepicted in FIG. 1A. As shown in steps B and C, a polymer 104 is spun(e.g., using a spin coater) and cured onto the conducting layer 102. Thepolymer 104 is cured using any suitable technique, such as simply roomtemperature or kiln (heated) drying or, for some types of polymer,ultraviolet curing. A non-limiting example of the polymer is Eccosorb®as produced by Laird. Traditionally, UV curing would not work onEccosorb® because it is not transparent. However, in thin enough layers,the polymer becomes transparent. Further, an additive can be used tomake cross-linking via UV possible. Alternately, ferrous nano-particlescan be added to a traditional photo resist to make such a material.

Spinning the polymer can be performed, for example, at the speeds andtemperature as illustrated in the plot as shown in FIG. 7 . FIG. 7 is aplot used to determine a first pass at spin speed (resist series) andtemperature in Celsius, depicting the resulting thickness and viscosityof the spun polymer. The spin-speed lookup plot of FIG. 7 depicts thethickness of two conventional photoresists (i.e., SU and AR) as afunction of their viscosity (i.e., their dilution in a solvent) and (forthe SU resist) as a function of spin speed. Also shown is the viscosityof CR-110 (CR-110 and CR-124 are examples of specific versions of theEccosorb® absorbing product) as measured at various temperatures. Forexample, if CR-110 were heated to 40 C, the spin process achieves a filmthickness of 10-30 microns, depending on spin speed. The spinningprocess has been shown to generate the desired spun polymer. Forexample, FIG. 8 is an scanning electron microscope (SEM) cross-sectionalimage depicting a Si wafer 800 with approximately 70 micrometers ofpolymer 802 (e.g., Eccosorb®) spun onto it, with approximately 300nanometers of aluminum 804 on top of the polymer 802. Thus, using theprocess as described herein allows for precise and thin dimensions ofthe polymer, etc. For example, in one aspect, at least one of, or both,of the first and second polymer dielectric layers have a thicknessbetween 0.1 and 1000 microns (or more desirably, between 1 to 100microns). Further, ground plane has a thickness between 0.1 and 10microns (or more desirably, between 0.5 to 3 microns). Additionally, themicrostrip signal line has a width between 0.1 and 1000 microns (or moredesirably, between 1 to 100 microns).

FIG. 9A further illustrates the spin rules, providing cross-sectionalresults from a profilometer scan (of a different wafer from thatdepicted in FIG. 8 ), showing an Eccosorb® step height of approximately3 microns. FIG. 9B is a table summarizing the two-dimensional surfaceparameters of the spun polymer.

Referring again to FIG. 1A. Step D involves applying a photo resist,patterning it, developing it, depositing a conductor (a metal such ascopper (cu)), then lifting off the resist, leaving behind patternedcopper. These are condensed into one graphical step, which will bereadily understood by those skilled in the art.

A hard mask 108, such as an aluminum hard mask, is then applied andpatterned to cover the conductor 106 and covered portions 110 of thepolymer 104, while leaving exposed portions 112 of the polymer 104exposed. Thereafter, the exposed portions 112 (cured polymer) are etchedaway using any suitable technique, such as a hot solvent or with a dryplasma etch. For example, for Eccosorb® CR, methyl ethyl ketone (MEK)can be used as a solvent. Regarding plasma etching, any plasma etch thatis chiefly ablative (SF₆ or Ar) can be used.

The hard mask (e.g., Al) 108 is then etched away to expose the conductor(e.g., Cu) 106 and covered portion 110 of the cured polymer 104. Forexample, Potassium Hydroxide (KOH) or Tetramethylammonium Hydroxide(TMAH) can be used to etch the Al layer while not etching the Cu layer.Step H involves application of photo resist, exposing it, developing it,then etching the now-exposed copper ground plane. In FIG. 1A, the groundplane (i.e., conducting layer 102) is deposited in Step B, and thesignal traces (i.e., conductor 106) are deposited in Step D. Theresulting devices may be diced or cleaved, then packaged for connectionor integration using standard IC techniques.

In another aspect, additional patterned conducting or absorbing layersmay be added. This refers to a notion of iterating either of thepreviously described processes, resulting in additionally stacked layersof the device. FIG. 4 depicts such a device, where additional dielectricand conducting layers have been added. FIG. 1B on the other hand,illustrates a single-layer product (e.g., micro-transmission line),which could have been produced by either of the discussed processes.

FIG. 1B depicts a model of a section of micro-strip transmission line ofarbitrary length terminated on either end with matched loads. The bottomrectangle in the X-Y plane is the microstrip ground plane 120A, and thenarrow thin rectangle above the ground plane is the microstrip signalline 120B. FIG. 1B depicts the microstrip transmission line filter 120with a CR-124 dielectric 120C. The vertical rectangles on either end arelumped-element 50-Ohm ports 122. A CR-124 dielectric 120C is an exampleof a specific version of the Eccosorb® absorbing product. FIG. 1Brepresents a zoom-in of a final product, such as Step D or H of FIG. 1A.In this illustration, the limited extent of the dielectric and of thelower copper layer is not shown. For further understanding, FIG. 1Cprovides a cross-sectional view of Step H of FIG. 1A as applicable to atransmission line filter 120, showing the components of Step H beingused as a microstrip ground plane 120A, the microstrip signal line 120B,and dielectric 120C. Note that FIG. 1C is not at the same scale as FIG.1B. Dimensions have been adjusted to promote conceptual clarity.

Another example of a transmission line filter is shown in FIGS. 4A and4B. In this example, the transmission line filter 400 has a striplinegeometry with vias 402 using CR-110 as a dielectric. The cylinders oneither side of the transmission line are the vias 402 that electricallyconnect the two ground planes 406 and 408. Here the “ground planes”refer to the initial bottom conducting layer, as shown in FIG. 1A aselement 102, as well as an additional large (third) conducting layer.This is the definition of a “stripline geometry.” In other words, thevias 402 connect the top and bottom conducting layers 406 and 408. Thecenter conductor 404 is the signal line and it is between the two groundplanes 406 and 408. The phrase “ground planes” is not requisite, but isunderstood by those skilled in the art. The rectangles on either end ofthe construction are 50-Ohm lumped ports 410

Alternately, one may use the above techniques to integrate a quantity ofpatterned Eccosorb® in a targeted manner to existing integrated circuitdesigns during fabrication.

Two simulations of such planar filters have been performed using AnsysHFSS, produced by Ansys, located at Southpointe, 2600 Ansys Drive,Canonsburg, Pa. 15317. Ansys HFSS is a 3D electromagnetic (EM)simulation software for designing and simulating high-frequencyelectronic products, such as the designs as depicted in FIGS. 1B and 4A.In this example, both are one centimeter (cm) long transmission lines.One with microstrip geometry (as shown in FIG. 1B) and CR-124 dielectrictargeting a 3 dB cutoff of 500 MHz (with results depicted in FIGS. 2 and3 ), and one with the strip line geometry (shown in FIG. 4A) using aCR-110 dielectric targeting a 3 dB cutoff of 5 GHz (results depicted inFIGS. 5 and 6 ). “Good reflection coefficient” was defined to mean theamount of reflected power is below −20 dB with reference to the incidentpower.

Finally, while this invention has been described in terms of severalembodiments, one of ordinary skill in the art will readily recognizethat the invention may have other applications in other environments. Itshould be noted that many embodiments and implementations are possible.Further, the following claims are in no way intended to limit the scopeof the present invention to the specific embodiments described above. Inaddition, any recitation of “means for” is intended to evoke ameans-plus-function reading of an element and a claim, whereas, anyelements that do not specifically use the recitation “means for”, arenot intended to be read as means-plus-function elements, even if theclaim otherwise includes the word “means”. Further, while particularmethod steps have been recited in a particular order, the method stepsmay occur in any desired order and fall within the scope of the presentinvention.

What is claimed is:
 1. A dissipative stripline transmission line filter,comprising: a microstrip ground plane, the microstrip ground plane beinga patterned conducting layer formed on a wafer substrate; a firstpolymer dielectric layer having magnetic nanoparticles formed on theground plane; a microstrip signal line formed on the polymer dielectric;a second polymer dielectric having magnetic nanoparticles formed on thesignal line and first polymer dielectric layer; a second ground planebeing a patterned conducting layer formed on the second polymerdielectric layer; and wherein each of the first and second polymerdielectric layers have a thickness between 0.1 and 1000 microns.
 2. Thefilter as set forth in claim 1, wherein each of the first and secondpolymer dielectric layers have a thickness between 1 to 100 microns. 3.A dissipative stripline transmission line filter, comprising: amicrostrip ground plane, the microstrip ground plane being a patternedconducting layer formed on a wafer substrate; a first polymer dielectriclayer having magnetic nanoparticles formed on the ground plane; amicrostrip signal line formed on the polymer dielectric; a secondpolymer dielectric having magnetic nanoparticles formed on the signalline and first polymer dielectric layer; a second ground plane being apatterned conducting layer formed on the second polymer dielectriclayer; and wherein the ground plane has a thickness between 0.1 and 10microns.
 4. The filter as set forth in claim 3, wherein the ground planehas a thickness between 0.5 to 3 microns.
 5. A dissipative striplinetransmission line filter, comprising: a microstrip ground plane, themicrostrip ground plane being a patterned conducting layer formed on awafer substrate; a first polymer dielectric layer having magneticnanoparticles formed on the ground plane; a microstrip signal lineformed on the polymer dielectric; a second polymer dielectric havingmagnetic nanoparticles formed on the signal line and first polymerdielectric layer; a second ground plane being a patterned conductinglayer formed on the second polymer dielectric layer; and wherein themicrostrip signal line has a width between 0.1 and 1000 microns.
 6. Thefilter as set forth in claim 5, wherein the microstrip signal line has awidth between 1 to 100 microns.
 7. A dissipative stripline transmissionline filter, comprising: a microstrip ground plane, the microstripground plane being a patterned conducting layer formed on a wafersubstrate; a first polymer dielectric layer having magneticnanoparticles formed on the ground plane; a microstrip signal lineformed on the polymer dielectric; a second polymer dielectric havingmagnetic nanoparticles formed on the signal line and first polymerdielectric layer; a second ground plane being a patterned conductinglayer formed on the second polymer dielectric layer; wherein each of thefirst and second polymer dielectric layers have a thickness between 1 to100 microns; wherein the ground plane has a thickness between 0.5 to 3microns; and wherein the microstrip signal line has a width between 1 to100 microns.